The present invention relates to an apparatus for molding a honeycomb structure, a spinneret for extrusion, and a method for molding a honeycomb structure.
Problems which become afresh are influences on earth environments and ecosystems of nitrogen oxides, sulfur oxides, hydrogen chloride, etc. contained in exhaust gases of automobiles in addition to carbon dioxide which warms the earth. It is said that, in the future, automobiles which utilize as energy source, for example, electricity, natural gas or methanol which have less influences on the ecosystem will be substituted for automobiles which utilize gasoline and gas oil as energy source. However, there are difficulties to overcome for practical use of these technologies, and tentative measures are reduction of cost of fuels and purification of exhaust gases. The honeycomb structures are used for exhaust gas purification apparatuses of automobiles as a technology for the solution of environmental problems.
For example, honeycomb structures made of ceramics materials or metallic materials as shown in FIGS. 2-4 are used as catalyst carriers for exhaust gas purification apparatuses of automobiles. FIG. 3 is an oblique view of a honeycomb structure 61, FIG. 4 is a front view of the honeycomb structure 61, and FIG. 2 is a partially enlarged view of the honeycomb structure 61. This honeycomb structure 61 is generally columnar as shown in FIG. 3, and comprises partition walls 64 having a honeycomb structure which form many cells 63 and an outer wall 62 covering the outer periphery.
For making such honeycomb structure 61, an apparatus for molding honeycomb structures, for example, as shown in FIG. 5, is used. FIG. 5 is a vertical sectional view of an apparatus for molding honeycomb structure which is provided with back pores 53 from which a molding material is introduced, a spinneret 54 having slits 52 through which the molding material is extruded, and a press plate 55 provided downstream the spinneret 54. Extrusion molding is carried out using this apparatus to make the honeycomb structure 61.
In the honeycomb structure molding apparatus 50, the spinneret 54 comprises an inner side part 71 and an outer periphery part 72, and the inner side part 71 protrudes to downstream side to form a level difference part 75 between the inner side part 71 and the outer periphery part 72. The inner side part 71 is provided with slits 73 by which a honeycomb structure is molded, and the outer peripheral part 72 is provided with slits 74 shorter than the slits 73. Furthermore, a gap part 57 which molds the outer wall of the honeycomb structure is provided between the spinneret 54 and the press plate 55. Press jig 58 and back press plate 59 are holders for setting the spinneret 54 and the press plate 55.
In carrying out extrusion molding by the honeycomb structure molding apparatus 50, the molding material is extruded from the upstream side of spinneret 54 to the downstream side through the spinneret 54 by an extruder (not shown) as shown in FIG. 5. The molding material extruded from slits 73 provided in the inner side part 71 of the spinneret 54 which opens on the downstream side forms a honeycomb structure comprising many cells 63.
On the other hand, the molding material extruded from slits 74 provided in the outer peripheral part 72 of the spinneret 54 flows with changing its running direction to the level difference part 75 from the extrusion direction with rupturing the honeycomb shape by the action of the gap part 57, and again flows with changing its running direction to the extrusion direction at the position where the press plate 55 opens, thereby forming an outer wall 62 surrounding the cells 63.
However, in such conventional honeycomb structure molding apparatus and molding method, there is the problem of failure in formation of cells.
As examples of failure in formation of cells, mention may be made of twist or rupture of cells as shown in FIG. 11. In the honeycomb structure molding apparatus shown in FIG. 5, there are problems that when the molding material for forming an outer wall flows with changing its running direction to the extrusion direction at the opening of the press plate 55, an excessive force is applied to the molding material for forming the cells in the outer peripheral part 72 to bring about twist of cells 94 or rupture of cells 93. Another example is omission of cells as shown in FIGS. 6 and 7. FIG. 6 is a front view of a honeycomb structure from which some cells are omitted, and FIG. 7 is an enlarged view of the portion in which omission of cells occurred in FIG. 6. Many cell omission portions 92 in the honeycomb structure 91 are seen in the direction of 90xc2x0 in the vicinity of the outer wall 62, and it is presumed that this phenomenon occurs because a molding material which is originally to form the partition wall 64 flows to the outer wall portion 62 at the time of being extruded from the spinneret.
The omission of cells means a phenomenon that one or more of partition walls which form cells 63 are not formed. The direction of 90xc2x0 at the section of the honeycomb structure means a direction progressing along the crossing partition walls 64 from nearly the center of the section of the honeycomb structure.
If failure in formation of cells occurs, strength of the honeycomb structure is deteriorated, and this is not preferred. Furthermore, when cells are closed in the form of checks and the honeycomb structure is used as a filter for apparatus for purification of exhaust gas of automobiles, there are portions which do not have filtration function to cause deterioration of performance. Moreover, when the honeycomb structure is used as catalyst carriers of automobile exhaust gas purification apparatus, recently, not only the exhaust gas purification performance is required, but also it is demanded to make the cell density as small as possible and the partition walls forming the cells as thin as possible for inhibition of deterioration of horse-power of automobiles. A honeycomb structure composed of a small number of cells thin in wall thickness is apt to be low in strength from the point of structure, and hence occurrence of failure in formation of cells which causes further deterioration of strength must be prevented by any means.
For the purpose of improving the strength of honeycomb structures, measures have hitherto been proposed.
For example, JP-A-57-157706 discloses various dies for honeycomb molding. FIGS. 8-10 are sectional views showing examples of the disclosed dies for honeycomb molding. Outer peripheral part of edge face of outlet side for a molding material of die 202 (spinneret) is cut out to form a step-like part 223 at the whole periphery, a level difference part 224 is provided between the central part of the edge face of outlet side for the molding material and the outer peripheral part of the die 202, and a ring space 204 is provided between the level difference part 224 and an inner peripheral face 231 of a die mask 203 (press plate) opposing and surrounding the level difference part 224.
In die 198 for honeycomb molding having a level difference part 224 of tapered angle as shown in FIG. 8, the molding material going in centripetal direction from molding channel 221 (slit) in the outer peripheral part of the die 202 is extruded after forcedly fed to the molding channel 221 in the central part of the die 202, and, hence, no pressure is applied to the portion near the peripheral wall (outer wall) of the already molded honeycomb body and no distortion is caused at the partition walls near the peripheral wall of the resulting honeycomb molded body. Furthermore, since the molding material filled in the space 204 flows together with the molding material coming straight from the molding channel 221 below the space 204 and is extruded along the inner peripheral face 231 of the die mask 203, a thick and dense peripheral wall is formed at the outer periphery of the resulting honeycomb molded body. Therefore, a honeycomb molded body of high strength can be obtained.
Furthermore, in a die 199 for honeycomb molding in which a space 205 is provided between an edge face 232 of the die mask 203 and the step-like part 223 of the die 202 which is opposite to the edge face 232 as shown in FIG. 9, density of the molding material fed under pressure from the molding channel 221 of the step-like part 223 and the space 205 to the molding channel of the central part and space 204 is smaller than in the case of using the die 198 for honeycomb molding, and, hence, the density of the molding material at the partition wall of piercing pores (cells) and the peripheral wall becomes uniform, whereby strength against heat shock can be improved.
Furthermore, in a die 200 for honeycomb molding in which the level difference part 223 is not tapered, but forms a face parallel with the inner peripheral face 231 of the die mask 203, and a space 204 is provided between these opposing faces as shown in FIG. 10, the same effects as in the case of the die 198 for honeycomb molding can be obtained.
However, there are no disclosures as to whether the failure in formation of cells as mentioned above, namely, twisting of cells, rupture of cells and omission of cells can be inhibited or not.
The object of the present invention is to provide a honeycomb structure molding apparatus according to which failure in formation of cells such as omission of cells can be inhibited, and, as a result, a higher strength can be given to even such honeycomb structure as having thin partition walls forming the cells and having a small cell density, and a molding method using said apparatus.
As a result of research conducted on honeycomb molding apparatuses, particularly, periphery of spinneret, for inhibiting occurrence of the above failure in formation of cells, it has been found especially effective to optimize the height of the level difference part of the spinneret. That is, if the height of the level difference part is too low, the molding material for the formation of outer wall gives an excessive force to the molding material for the formation of cells when they contact with and bond to each other to result in twist of cells and rupture of cells. If the height of the level difference part is too high, a part of the molding material for the formation of cells which is to be allowed to contact with the molding material for the formation of outer wall does not flows through the slits to cause omission of cells.
Therefore, according to the present invention, there is provided a honeycomb structure molding apparatus for making a honeycomb structure by extrusion molding, characterized in that it has at least a plate-like spinneret provided with back pores for introducing a molding material and slits for extruding the molding material, and a press plate for fixing the spinneret, the spinneret comprises an inner side part and an outer peripheral part, the inner side part protrudes to the downstream side to form a level difference part between the inner side part and the outer peripheral part, the press plate opens opposing the inner side part of the spinneret and presses the downstream side of the outer peripheral part of the spinneret, and width W (mm) of slit, length L1 (mm) of slit of the inner side part and length L2 (mm) of slit of the outer peripheral part satisfy the following two formulas.
L1xe2x88x92L2xe2x89xa70.4 mm 
L2xe2x89xa70.015/Wxc3x97L1 
In the honeycomb structure molding apparatus of the present invention, the level difference part is preferably right-angled with having an inclined plane of about 90xc2x0. The width W of the slits is preferably about 120 xcexcm or less.
The honeycomb structure molding apparatus of the present invention can be suitably used for molding a honeycomb structure having a cell density of about 2.0 cells/cm2 or less, and, besides, can easily mold a honeycomb structure having a thick outer wall of about 0.3 mm or more in thickness.
Moreover, the honeycomb structure made by the honeycomb structure molding apparatus of the present invention can be suitably used as catalyst carriers for purification of exhaust gases of automobiles.
According to the present invention, there is provided a plate-like spinneret for extrusion molding a honeycomb structure which is provided with back pores for introducing a molding material and slits for extruding the molding material, characterized in that the spinneret comprises an inner side part and an outer peripheral part, the inner side part protrudes to the downstream side to form a level difference part between the inner side part and the outer peripheral part, and width W (mm) of slit, length L1 (mm) of slit of the inner side part and length L2 (mm) of slit of the outer peripheral part satisfy the following two formulas.
L1xe2x88x92L2xe2x89xa70.4 mm 
L2xe2x89xa70.015/Wxc3x97L1 
Furthermore, according to the present invention, there is provided a honeycomb structure molding method for obtaining a honeycomb structure by extrusion molding a molding material containing at least a ceramics powder and/or a metallic powder and a molding aid, characterized in that there is used a spinneret jig for extrusion of a honeycomb structure which has a plate-like spinneret provided with back pores for introducing a molding material and slits for extruding the molding material, and a press plate for fixing the spinneret, the spinneret comprising an inner side part and an outer peripheral part, the inner side part protruding to the downstream side to form a level difference part between the inner side part and the outer peripheral part, the press plate opening opposing the inner side part of the spinneret and pressing the downstream side of the outer peripheral part of the spinneret, the level difference part being right-angled with an inclined plane of about 90xc2x0, and by specifying the height of the level difference part to be about 0.5-4 mm, the difference in passage resistance between the slits of the inner side part and those of the outer side part is reduced and thus failure in formation of cells can be inhibited.